Aluminum alloys are enjoying growing use as automobile parts and are rolled into sheets which may be stamped into hoods, trunk lids, doors, and fenders, and the like from the aluminum alloy sheet. At present, however, none of the existing aluminum alloys suitable for use in forming automobile panels and parts appears to satisfy the specifications of the various automotive companies, as the standards tend to differ from one company to the other. For example, one company's requirements may emphasize alloy strength (e.g., a yield strength in excess of 25 ksi), while other companies may prefer a softer alloy (e.g., a 15-18 ksi yield strength in the as delivered state), which has superior formability properties. Often, improvements in an alloy's formability decreases the ability of heat treatment of the alloy to improve its strength. As such, there exists a need for an alloy which may be formed easily into automotive body panels, but which has good age hardening properties so that when the alloy panels are heat treated, such as during the paint baking cycle, the strength of the alloy increases.
Various studies and previous attempts have been made to develop improved aluminum alloys which may be suitable for use in manufacturing automobile body panels, for example, and which have a composition displaying good age A hardening properties. For example, U.S. Pat. No. 4,838,958 (Komatsubara) discusses a T-4 tempered and straightened rolled sheet Al-Mg-Cu series aluminum alloy which according to the patentees contains from 1.5 to 5.5% by weight of magnesium and 0.18 to 1.5% by weight of copper as the essential alloying ingredients, in an effort to improve mechanical properties, formability, and to help avoid formation of Luder's marks. U.S. Pat. No. 4,589,932 (Park) appears to pertain to an alloy composition containing 0.4% to 1.2% Si, 0.5% to 1.3% Mg, 0.6% to 1.1% Cu, and 0.1% to 1% Mn. The patentee states that the alloy is responsive to high temperature artificial aging treatments.
In U.S. Pat. No. 4,637,842 (Jeffrey et al.), the patentees discuss a method for producing Al-Mg-Si alloy sheets and articles. The patentees, however, do not attempt to create phases in an effort to improve the age hardening properties of the alloy.
U.S. Pat. No. 4,113,472 (Fister) proposes an aluminum alloy containing 0.9 to 1.5% magnesium, 0.4 to 0.8% silicon, and 0.9 to 1.5% copper, which purports to give the alloy high strength, extrudability, and weldability.
However, the foregoing alloys require very close control over the natural and artificial aging cycle if appropriate combinations of strength and formability are to be achieved. In practice it is important that the T4 strength be relatively low, and the natural aging rate be slow, so that good formability can be maintained over a long period of time. Subsequently the alloy needs to show a high precipitation hardening response during the paint bake cycle so that a high final strength in the formed, painted part can be achieved.